Before that humanitarian groundswell called the animal rights movement emerged, a nutria pelt would sell for over $8, a well-tailored coat for a couple thousand. No wonder there were so many nutria around, bred on farms throughout the south and west. There was money in it.

Then the fickle god of fashion frowned, and the fur industry fell into a funk. Coast to coast thousands of caged nutria were rendered worthless. Nobody wanted their soft brown fur. Before long, nutria escapes became common, here and there hundreds made for the swamps. Hurricanes, floods, and other unpleasant expressions of Mother Nature's nature helped by blowing over fences and raising water levels, enabling the animals to swim to freedom. Some fur farmers simply opened the gates for their devalued inventory.

Time passed. Then came the blowback: the revenge of the nutria on America, the assiduous assault by an alien horde of orange-toothed ecological terrorists. The invasive nutria, because it consumes the roots of plants, has, over the years, destroyed millions of acres of marshland – the habitat that baby fish, crabs, ducks, and other fauna need to survive. Nutria wreak havoc just by doing what comes naturally to them.

What is the nutria? It's a brown, semiaquatic rodent, with a long ratlike tail, webbed hind feet, and orange beaverlike teeth. It comes from Patagonia, and grows to about 20 pounds and two feet in length. It lives two to three years; the female produces about 10 offspring a year.

Nutria are now established in about 40 states, up the east coast from Louisiana to the Mason-Dixon line, and out west from California to Washington. They were first introduced in the US in 1899, in California, then Louisiana in the 1930s. Their first big breakout occurred in 1941 during a hurricane in Louisiana. Today Louisiana is infested with about 20 million nutria, say wildlife authorities.

Affected states have tried different strategies: poison, trapping, and shooting. Louisiana pays a bounty of $4 for each nutria tail, and gets about 300,000 a year, says Steve Kendrot of the US Department of Agriculture's Wildlife Services. Another strategy was to encourage people to eat them. In 1998, Louisiana held a big nutria cook-off, inviting the state's famous chefs. That same year Maryland held its own bash in Dorchester County on the eastern shore of Chesapeake Bay, to attract support for nutria eradication.

Nick Carter, a conservationist known for his nutria recipes throughout the Delmarva Peninsula (which runs down the eastern shore of the Chesapeake through Delaware, Maryland, and Virginia), played chef. The menu, high toned as he could get it, included nutria en hoof de barbecue, medallions de nutria, nutria au vin, and Nick's Smokehouse Nutria.

"We all had a good time," Mr. Carter recalls. "You can do anything with the nutria: chili, stew, barbecue. They taste better than muskrat."

Sadly, he isn't cooking much these days: "There aren't many nutria around anymore." Also, the rodent proved less than a culinary hit. Maybe being better than muskrat just wasn't enough.

The consequences of the nutria's work in the Blackwater National Wildlife Refuge here in Dorchester County are visible: vast expanses of open water where once there was marsh, here and there ash-colored dead trees rising at cockeyed angles from the shallow reaches. In the late '80s about 40,000 nutria thrived here. Several frigid winters decimated that population, but not before the nutria, gobbling up roots, dislodging plants, and dispersing the soil that held them, had destroyed half the Blackwater's 13,000 acres of marsh.

In 2002, about 10,000 nutria held out within and around the Blackwater's marshlands, when Mr. Kendrot of the USDA launched the Nutria Eradication Program. By deploying experienced trappers to bait and trap over and over again, the animals were virtually eliminated. "Now there are only about a dozen," says Kendrot. "We have trouble finding nutria in the Blackwater Refuge."

Kendrot has degrees in wildlife management and ecology, a patient manner, and is given to second thoughts: Shortly after his initial claim, as we approached a pen containing several captive nutria (they looked shifty, suspicious, and fearful), he distanced himself from his estimate: "Don't quote me on a number. Let's just say they're effectively gone, reduced by 99 percent or more."

No one would be put off by a little boastfulness. In fact, a couple years back a report in a Washington newspaper declared the war against the nutria won in the Blackwater.

Kendrot is leery of breast beating; it might give his superiors the impression he has nothing to do. In fact, he has plenty to do: "We're poised to expand outward. Our mission is to eradicate nutria from all of Delmarva."

If the estimates of nutria infestation are accurate, his campaign in the refuge was just a skirmish compared with what lies ahead. The refuge may be nutria-free, but Dorchester County, which harbors the "nutria source population" for every place around it, is not.

The Virginia portion of Delmarva, he says, is "lightly populated." Delaware, too.

When I told him I had seen nutria in Delaware, his eyes brightened: "Maybe you saw muskrats." Then he asked where I saw them, so he might send someone to investigate.

When I returned to the Bethany Beach site in Delaware, I talked to the owner of an antique shop nearby. He said, "We've got nutria all around. They're everywhere."

Dan Murphy, a federal Fish and Wildlife Service biologist who's with the eradication program, says he's aware of nutria in Delaware. "They're in the state parks. We know they are there. We'll go to Delaware."

He estimates a campaign to rid the Chesapeake of every nutria would cost up to $4 million and require 40 more people, mostly trappers. He stresses the word eradication, not control. Control is what's happening in Louisiana, he says. With 20 million nutria, and where winters are too warm to kill off large numbers of nutria, a species that despite its century-long presence in North America has never learned to create a dwelling that would shield it from the cold, control is all they can do.

Mr. Murphy believes eradication is possible on Delmarva: "If we don't eradicate them, they'll be back after a few warm winters. We think we can."